This invention relates to cellular radio systems.
In a typical known cellular radio system a mobile radio unit in a cell communicates via a radio channel with the cell's base station, which in turn communicates with a fixed, land-based switch connected to the landline telephone system. When the mobile radio unit moves from one cell to another adjacent cell, a handover must be performed, in which the mobile ceases communicating with the old cell's base station on a first channel and begins communicating with the new cell's base station on a second, different channel. This handover process causes a disruption in communication to and from the mobile unit. This disruption is normally experienced in the form of an audio mute. Since the background noise is often quite high in a mobile environment, a mute is usually quite obvious. The coverage area of cellular radio systems heretofore has been such that the cell size has been sufficiently large to produce only occasional mutes which are not objectionable to most users. However, as cells shrink in size (as they are expected to do with digital cellular radio systems) handovers will occur much more frequently and the consequent mutes will become proportionately more disruptive to communication.
In existing digital cellular systems it is known to employ a "fill-in" function in the event of missing speech blocks or as a result of decoding errors or "stolen" speech blocks used for data. In such a known system a "fill-in" audio signal is extrapolated (using one of a variety of, known algorithms) from an immediately preceding audio signal for a period of up to some 320 milliseconds to fill in for missing audio.